Differential expression of S100 beta and glial fibrillary acidic protein in the hippocampus after kainic acid-induced lesions and mossy fiber sprouting in adult rat

The expression of S100 beta and glial fibrillary acidic protein (GFAP) was analyzed following bilateral injection of kainic acid (KA), a glutamate der ivative, into the CA3 region of the adult rat hippocampus, This treatment p roduces a progressive degeneration of the pyramidal neurons of the hippocam pus while sparing the granule cells of the dentate gyrus which undergo spro uting of their axons in the supragranular layer. Messenger RNA and protein levels were measured, by Northern blot and ELISA, in the hippocampus of les ioned and sham-operated rats I, 7, and 30 days after KA injection, A signif icant increase of GFAP and its mRNA was demonstrated at each time point, wh ereas S100 beta mRNA levels were significantly enhanced only 30 days after the KA injection and the levels of S100 beta protein remained unchanged at all time points. However, when analyzed by immunohistochemistry the S100 be ta showed clear changes in its expression and distribution depending on the region considered. One month after KA injection, S100 beta immunoreactivit y was considerably reduced in the stratum radiatum of CA3 region, but there was increased S100 beta immunoreactivity in the stratum moleculare. In par ticular, a notable band of S100 beta positive, hypertrophic astrocytes appe ared in the supragranular layer of the dentate gyrus where the sprouting of mossy fiber collaterals was detected by Timm's staining. These data show f or the first time that an increase in S100 beta expression in subpopulation s of reactive astrocytes may be involved in the structural reorganization o f the hippocampus following KA-induced neurodegeneration, (C) 2000 Academic Press.